Method for dimensional x-ray measurement, in particular by computed tomography, and x-ray computed tomography scanner

Abstract

The invention relates to a method for dimensional measurement by way of X-ray computed tomography, featuring the steps (a) Irradiating a test object (26) with non-monochromatic X-ray radiation from a virtually punctiform X-ray source (12), (b) measuring the intensity (I) of the X-ray radiation (22) in the radiation path behind the test object (26) by means of a detector (14) which has a plurality of pixels (P) to obtain pixel-dependent intensity data (I(P)), and (c) calculating at least one dimension (H) of the test object (26) using the pixel-dependent intensity data (I(P)). According to the invention, the pixel-dependent intensity data (I(P)) is corrected by the influence of an effective penetration depth (τ) on the detector and/or a displacement of the effective source location (Q) on a target (20) of the X-ray source (12).

Claims

1. A method for dimensional X-ray measurement, comprising: (a) irradiating a test object with non-monochromatic X-ray radiation from a virtually punctiform X-ray source, (b) measuring an intensity of the X-ray radiation in a radiation path behind the test object by a flat detector which has a plurality of pixels to obtain pixel-dependent intensity data, and (c) calculating at least one dimension of the test object using the pixel-dependent intensity data, (d) correcting the pixel-dependent intensity data based on an effective penetration depth change on the flat detector due to beam hardening and/or a displacement of an effective source location change on a target of the X-ray source due to beam hardening.

2. The method according to claim 1 wherein the X-ray radiation is generated by irradiating a source point of a target with electrons, (i) calculation of dimensions of the test object is executed using an enlargement factor, which depends on a distance of the source point from the detector and a distance of the source point from the test object, and that (ii) the enlargement factor is corrected based on an X-ray emission spectrum that changes due to a changing electron penetration depth into the target.

3. The method according to claim 1 wherein the X-ray radiation is generated by irradiating a source point of a target with electrons such that raw radiation occurs, and further comprising filtering of the raw radiation by a filter, said filter filtering out a maximum of 75% of a total intensity of the raw radiation.

4. An X-ray computed tomography scanner, comprising: (a) an X-ray source for generating X-ray radiation, (b) a detector, which features a plurality of pixels, for measuring pixel-dependent intensity data of the X-ray radiation, (c) a movement device for moving a test object relative to the X-ray source and the detector, and (d) an evaluation unit for calculating a three-dimensional image of the test object using the pixel-dependent intensity data, wherein the evaluation unit is designed to automatically execute a method according to claim 1.

5. The X-ray computed tomography scanner according to claim 4, wherein the evaluation unit is designed to automatically execute a method containing the steps: correcting pixel-dependent intensity data based on a penetration depth on the detector and/or a displacement of the effective source location on a target of the X-ray source, such that corrected pixel-dependent intensity data is obtained, and calculating the three-dimensional image using the corrected pixel-dependent intensity data.

6. The X-ray computed tomography scanner of claim 4 wherein the movement device is a rotation device.

7. A method for dimensional X-ray measurement, comprising: (a) irradiating a test object with non-monochromatic X-ray radiation from a virtually punctiform X-ray source, (b) measuring an intensity of the X-ray radiation in a radiation path behind the test object by a flat detector which has a plurality of pixels to obtain pixel-dependent intensity data, and (c) calculating at least one dimension of the test object using the pixel-dependent intensity data, (d) correcting the pixel-dependent intensity data based on an effective penetration depth on the flat detector and/or a displacement of an effective source location on a target of the X-ray source wherein the pixel-dependent intensity data is corrected by a change in the effective penetration depth on the detector, said change being caused by beam hardening.

8. A method for dimensional X-ray measurement, comprising: (a) irradiating a test object with non-monochromatic X-ray radiation from a virtually punctiform X-ray source, (b) measuring an intensity of the X-ray radiation in a radiation path behind the test object by a flat detector which has a plurality of pixels to obtain pixel-dependent intensity data, and (c) calculating at least one dimension of the test object using the pixel-dependent intensity data, (d) correcting the pixel-dependent intensity data based on an effective penetration depth on the flat detector and/or a displacement of an effective source location on a target of the X-ray source, wherein the pixel-dependent intensity data is corrected by a displacement of the effective source location on the target of the X-ray source, said change being caused by beam hardening.

9. A method for dimensional X-ray measurement, comprising: (a) irradiating a test object with non-monochromatic X-ray radiation from a virtually punctiform X-ray source, (b) measuring an intensity of the X-ray radiation in a radiation path behind the test object by a detector which has a plurality of pixels to obtain pixel-dependent intensity data, and (c) calculating at least one dimension of the test object using the pixel-dependent intensity data, (d) correcting the pixel-dependent intensity data based on an effective penetration depth on the detector and/or a displacement of an effective source location on a target of the X-ray source, wherein the correction by a change in the effective penetration depth comprises the following steps: for at least a majority of the pixels (i) identifying a zero point distance of a pixel from an optical axis, (ii) identifying an intensity of the X-ray radiation measured by the pixel, (iii) allocating a corrected position depending on the zero point distance and the intensity for the pixel, and (iv) calculating corrected pixel-dependent intensity data from all corrected positions and the corresponding intensities.

10. The method according to claim 9, wherein a corrected position and a zero point distance of an original position lie on one line.

11. The method according to claim 9 wherein a differential distance between the zero point distance of a corrected position and a zero point distance of an original position is calculated from a term which contains a product of a function of the intensity, an intensity correction parameter and a constant.

12. The method according to claim 11, comprising: measuring a test object in the form of a calibration body, changing a filter strength of a filter, and calculating an intensity correction parameter from a displacement of a shadow image of the test object on the detector, depending on the intensity of the X-ray radiation.

Description

(1) In the following, the invention will be explained in more detail by way of the attached drawings. They show

(2) FIG. 1 a schematic depiction of an X-ray computed tomography scanner according to the invention for conducting a method according to the invention,

(3) FIG. 2 by way of the partial FIGS. 2a and 2b, a schematic depiction of how the pixel-dependent intensity data is corrected by the factor to compensate for the influence of the penetration depth, and

(4) FIG. 3 by way of the partial FIGS. 3a and 3b, experimental data in which the dependency of the enlargement on the absorbed intensity is shown.

(5) FIG. 1 shows a schematic view of an X-ray computed tomography scanner 10 according to the invention which comprises an X-ray source 12 and a detector 14. The X-ray source 12 has an electron beam source 16 for generating an electron beam 18, which is directed at a target 20. The target 20 is made of tungsten, for example. The electrons of the electron beam 18 have an energy of 225 kilo electronvolts, for example. The electron beam 18 strikes the target 20 at a source point Q. An angle of impact between the electron beam 18 and a surface of the target 20 preferably lies between 15 and 30°. Alternatively, a thin target can may also be penetrated from behind.

(6) When the electrons of the electron beam 18 strike the target 20. X-ray radiation occurs, which, for the purposes of a simplified assessment, can be considered to be made up of several X-ray beams 22.i. The drawing shows two beam paths with the index i=1, 2. An optical filter 24 is arranged in the beam direction behind the target 20, said filter effecting beam hardening of the X-ray beams 22.i. The filter is made of aluminium or copper, for instance, and has a thickness d.

(7) A test object 26 is arranged in the beam path behind the filter 24. The test object 26 comprises a structure 28 that is to be measured, such as a bore, and the material 30 surrounding the structure 28. This imagined division of the test object 26 into structure 28 and material 30 only serves to explain the invention and should not contain any restrictive statements on the type of test object.

(8) The X-ray computed tomography scanner 10 preferably features a test specimen accommodation for accommodating the test object 26. The test specimen accommodation is preferably designed as a movement device 32, especially a rotation device for rotating the test object 26 about a rotational axis D. The rotational axis D is at a first distance a from the source point Q.

(9) The detector 14 is arranged in the beam direction behind the test object 26 and is at a second distance b from the source point. In the present case, the detector 14 features a scintillator element 34, which has a plurality of microcolumns 36. The microcolumns extend perpendicular to a detector plane E and are made of cesium iodide crystallite needles, for example. If an X-ray quantum strikes the detector 14, a flash of light appears, which then spreads along the adjacent microcolumns, thereby striking a small number of photovoltaic cells. It should be noted that the running Index I is used for several objects without it specifically referring to an assignment of said objects with respect to each other.

(10) FIG. 1 depicts two scenarios for a shadow image S, S′ of the test object 26 on the detector 14. For the first scenario, two X-ray beams 22.1′, 22.2′ are depicted with a dashed line leaving the source point Q′; this corresponds to the case in which no filter 24 is present and the test object 26 is made up solely of the structure 28. This results in minimal beam hardening and a higher intensity, which is measured by the detector 14. Due to the low degree of beam hardening, the effective penetration depth τ′ is comparatively small. For the second scenario, two X-ray beams are depicted by a solid line, said beams passing through the same pairs of points P1 and P2 of the structure, wherein a filter 24 is available and/or the structure 28 is surrounded by a significant amount of material 30. As detailed in the introduction to the description, beam hardening occurs and thus also a greater effective penetration depth T into the detector 14. The difference δ=τ−τ′ is greater than zero. The path from the source to the detector b thus increases by this value and, at a ratio (b+δ)/b, the depiction of the points P1 and P2 lies further apart than in the original image. A second impact of the additional beam hardening effected by the absorber 30 is that the average source location Q on the target is different. Given that the target runs diagonally to the axis A, the distances a and b enlarge simultaneously by the value τ.sub.e; a lateral offset occurs, which strikes in a geometrically enlarged form as offset E in the image in the direction of the target tilt. The measurement results of the detector 14 are evaluated by means of an evaluation unit 40, which comprises at least processor and a digital memory for this specific purpose.

(11) FIG. 2a shows how the described effect can be corrected and schematically depicts a section of the detector 14 with the pixels P.sub.x,y. The pixel P.sub.2,3 detects a very low intensity I.sub.2,3=I(P.sub.2,3). Conversely, the pixels P.sub.3,3 and P.sub.2,2 detect a median intensity I.sub.3,3 or I.sub.2,2. For the remaining pixels P, it is assumed that, for the sake of simplicity, they detect a maximum intensity I.sub.x,y=I.sub.max.

(12) The pixel P.sub.2,3 has a zero point distance r.sub.2,3 from a zero point N (see FIG. 1) of the detector 14 on the optical axis (see FIG. 1). The optical axis A is the line that runs through the source point Q and stands perpendicular to the detector plane E along which the detector 14 extends. Approximately, the source point is used that is detected when neither a filter nor a test object is present in the structure.

(13) FIG. 2b shows that the pixel-dependent intensity data can be corrected by the influence of the penetration depth τ and the source location displacement τ.sub.e by allocating the intensity I(P.sub.2,3)), i.e. the intensity measured by the pixel P.sub.2,3, a new position K. This new position K is calculated by way of the displacement of the position of the original pixel P.sub.2,3 in the direction of the connection line L.sub.2 from the optical axis A to the original position of the pixel P.sub.2,3. The new distance r′.sub.2,3 is calculated as r′.sub.2,3=r.sub.2,3(1+k.Math.i.sub.2,3+c) with the displacement parameter k, the material-dependent constant c and the intensity I. For the sake of simplicity, it is assumed here that c=0.

(14) As schematically depicted in FIG. 2b, this corresponds to an imagined displacement of the position of the pixel P.sub.2,3 relative to the original pixel pattern and thus the original coordinate system. In the same way, the corresponding displacement is calculated for all pixels P.sub.x,y. This is implied for the pixels P.sub.3,3 and P.sub.2,2.

(15) In a subsequent step, each pixel P.sub.x,y is allocated a corrected intensity I′.sub.x,y. This is achieved by calculating for each pixel what proportion of the surface of the respective pixel exhibits the calculated displaced intensity. In this way, the intensity I′.sub.2,3 is allocated to the pixel P.sub.2,3, which in the present case corresponds to 0.52-times the intensity I.sub.2,3, given that only 52% of the black surface lies in the region of the pixel P.sub.2,3, which can be seen in FIG. 2b. This area B is circled with a dot-dash line in FIG. 2b. The intensity I′.sub.3,3, which is allocated to the pixel P.sub.3,3, is I′.sub.3,3=I.sub.2,3.Math.0,22+0,82.Math.I.sub.33. This calculation is conducted for all pixels P.sub.x,y of the original image of the detector 14. The intensity data that is corrected in this manner produces a corrected image of the detector 14 and is then used to reconstruct a three-dimensional density image of the test object 26.

(16) It should be noted that it is beneficial, but not necessary, to calculate the intensities for the original pixel pattern. It is also conceivable and included in the invention that this pixel-by-pixel intensity correction, which redistributes intensity to other adjacent pixels, be applied for a different pixel pattern, such as hexagonal lattice.

(17) FIG. 1 shows that an enlargement factor V=b/a can be calculated from the first distance a and the second distance b, said enlargement factor indicating how much larger the shadow image S, S′ appears on the detector 14 in relation to the structure 28. In order to measure a dimension, such as a height H of a recess in the test object 26, this enlargement factor V must be known. The enlargement factors V1 to V4 and the distances a and b are explained above. The enlargement is an indirect measured value if the dimensions of the calibration lattice are known and the pixel distances of the detector are assumed to be known (e.g. 200 μm). Firstly, these are very well-known and secondly, the detector pixel sizes are no longer included in the results, as all dimensions are measured in pixels/voxels and considered in relation to the calibration object.

(18) The X-axis in FIG. 3a is the intensity I measured by the detector. The intensity is changed by way of increasingly thicker pre-filters made of either copper (circles) or aluminium (squares). The test object 26 consists solely of a structure 28 in the form of an aluminium foil with several recesses, which are arranged at known positions. The Y-axis indicates the enlargement factor V. It should be noted that the enlargement factor V decreases with increasing intensity I or increases with stronger absorption. The reason for this change is the above-described influence of the increasing beam hardening on the penetration depth and the apparent position of the source point.

(19) As in FIG. 3a, FIG. 3b depicts a diagram, wherein the test object 26 consists solely of a structure 28 in the form of a copper foil with several recesses, which are arranged at known positions.

(20) TABLE-US-00001 Reference list 10 X-ray computed tomography scanner 12 X-ray source 14 detector 16 electron beam source 18 electron beam 20 target 22 X-ray beam 24 filter 26 test object 28 structure 30 material 32 test specimen accommodation, rotation device 34 scintillator element 36 micro-columns 38 photovoltaic element 40 evaluation unit a first distance A optical axis b second distance d filter strength D rotational axis E detector plane H height I intensity K position L distance to centre of image P pixel Q source point r distance S shadow image k intensity correction parameter V enlargement factor w constant δ difference ε displacement τ effective penetration depth τ.sub.e effective source point Displacement